1. Field of the Invention
This invention relates to a device for observing a light signal.
2. Description of the Prior Art
A light signal for use in light communications is usually intensity-modulated; but it is difficult to accurately observe what level or modulation degree the light has.
For converting a light signal into an electrical signal use is generally made of a PIN diode, an avalanche photo diode (APD) or the like. In such photo detectors for detecting light, a current when no incident light exists, that is, a dark current changes with various conditions, such as temperature, aging, a power source voltage and so forth; this degrades the accuracy of measurement most.